On Sept. 1st, 2009, the NYU Health Sciences Libraries made the unprecedented move from their Millennium ILS to Koha. The migration was done over the course of 3 months, without assistance from either Innovative Interfaces, Inc. or any Koha vendor. The in-house script, written in Perl and XSLT, can be used with any Millennium installation, regardless of which modules have been purchased, and can be adapted to work for migration to systems other than Koha. Helper scripts were also developed to capture the current circulation state (checkouts, holds and fines), and do minor data cleanup. This presentation will cover the planning and scheduling of the migration, as well as an overview of the code that was written for it. Opportunities for systems integration and development made newly available by having an open source platform are also discussed.